“Everything in moderation” has long been popular go-to diet advice, suggesting there is little harm in small doses of unhealthy foods. However, this approach may actually contribute to a higher likelihood of developing metabolic illness such as diabetes and obesity, according to new research.

Obesity has become an epidemic in the United States: According to the CDC, more than one-third—34.9% or 78.6 million—of U.S. adults are obese, and 29.1 million Americans have diabetes, 1.25 million of whom are children. Now, researchers at the University of Texas Health Science Center at Houston (UTHealth) and the Friedman School of Nutrition Science and Policy at Tufts University have published a study in PLOS ONE that shows how the "moderation" rule in dieting may be adding to these problems.

Lead author Marcia C. de Oliveira Otto, assistant professor of epidemiology at UTHealth, was curious about the origins of this moderation-diet advice, and began to investigate how food diversity relates to, and affects, metabolic health. Using data from the Multi-Ethnic Study of Atherosclerosis, her team assessed diet diversity among 5160 study participants.

“This idea of ‘a little bit of everything is good for you' comes from studies looking at populations with restricted numbers of foods, like people in Africa who get their calories from a limited number of starch foods,” Otto tells mental_floss. “Nowadays, in developed countries, we live in a context of [an] abundance of foods.” Not all of these foods are good for us.

Before this study, the only existing metric for the relationship between food diversity and metabolic health was the number of different foods people eat in a certain period of time. To figure out how “a little bit of everything” translated into a dietary pattern in a country like the U.S., Otto’s team added two more metrics to food count: evenness (how much caloric intake is distributed across different foods) and how alike, or different, the food items are. “That last is the first time this metric has been used to evaluate diet,” Otto says. “We found that people eating a greater number of foods were eating equal numbers of healthy and unhealthy foods.”

The team hypothesized that a greater intake of healthy foods was being outweighed by the potential harm from less healthy foods. “So we studied dissimilarity, looking at those who eat everything from broccoli to soda and candy," she says. "By thinking that everything in moderation is a good thing, we are giving people the green light to eat more unhealthy foods, and then these [unhealthy foods] displace healthy foods in diets.” In metabolic outcomes, people who ate this way had a greater increase in weight circumference—an indicator of obesity—in five to 10 years.

Furthermore, the team investigated the issue of food quality—how healthy and nutrient-rich the foods were— people ate using the Dietary Approaches to Stop Hypertension (DASH) score and the Alternative Healthy Eating Index (AHEI or HEI) score. “At 10 years, we found people with greater food quality had 25% lower risk of developing type 2 diabetes,” Otto says. 

To their surprise, participants with more food diversity (dissimilarity) had worse diet quality, as though the variety gave them permission to eat more unhealthy foods. She points to the inundation of processed and packaged foods in grocery stores as a factor in people’s choices. “Consuming more trans fats, sodium, and sugar are associated with processed-food consumption and poor metabolic health, mainly diabetes and obesity,” she says.

“This research gives us an opportunity to rethink a concept that is widely spread,” Otto says. So perhaps rather than thinking “everything in moderation," she notes, “we should stick to eating what we know is healthy.”